Hermetic seal



y 1941- 1 LmbwuRM ETAL 2,251,062

HERMETI C SEAL Filed Oct. 28, 1940 lnven kcjrsz Leopold Lindvvur'm,

oL'F an Thouret b I WW Their" At'torneg.

Patented July 29, 1941 UNITED STATES PATENT OFFICE HERMETIC SEAL LeopoldLindwurm, Berlin,

and Wolfgang Thouret, Berlin-Wilmersdorf, Germany, assignors to GeneralElectric Company, a corporation of New York Application October 28,1940, Serial No. 363,190

- In Germany'octoher 14, 1939 2 Claims. (CL 176-126) The presentinvention relates to hermetic, electrically conducting seals for gaseouselectric discharge devices and more particularly to seals 1 such as ametal foil. fused into a vitreous body having a low coemcient ofexpansion.

- comprising a thin, flat. elongated metal member,

The vitreous part of the seals to which the pres:

ent invention'pertains is slender and cylindrical in shape and the thinmetal foil or strip is mounted with its largest dimension in or parallelto the longitudinal axis of the vitreous seal part. A pair of wiresmounted in the same straight line in or parallel to the longitudinalaxis of the vitreous part and passing through said .foil or strip isattached to the edge portions oi the foil in opposingpositions. Saidwires terminate beyond the respective ends of the vitreous seal part.The distance between the, ends of the wires fastened to the strip isappreciable.

Seals of this type are useful for gaseous electric discharge lampshaving a quartz or hard glass envelope containing a starting'gas and aquantity of mercury the vapor of which is at a pressure of about 10atmospheres or higher during the operation of the lamp. Lamps of thistype are eflicient light sources of high intrinsic brilliancy.

The seals of these high pressure discharge lamps I must be strongmechanically and resistant to heat to remain gas tight under highpressure.of the vapor atmwphere and thehigh temperature The seal usuallyprotrudes from the envelope. Frequently the seal becomes so hot inservice that the part of the metal conductor external to the seal andexposed to the oxygen containing atmosphere is oxidized. ;The oxidelayer thus formed gradually creeps along the conductor and into thefused joint between the metal and the vitreous parts of the seal. TheJoint then becomes pervious to gas to terminate the useful life of thelamp. I

The formation of an oxide layer on the exposed part of the electricalconductor of the seal occurs only at elevated temperatures when saidpart consists of tungsten or molybdenum, for

example. If the exposedpart of the conductor isbelow the oxidizingtemperature thereof during the operation of the lamp dimculties causedby the oxide layer are avoided. Increasing the heat dissipating capacityof the seal, such as by plac-, ing heat radiating fins-incontacttherewith, or increasing the length of the seal, are helpful butnot completely effective in maintaining the exposed part of theconductor below the oxidizing temperature thereof. Such structures aredisof the envelope during the operation of the lamp.

advantageous since the mounting and transportation thereby.

"The object of the present invention is to pro-. vide a mechanicallystrong. hermetic, electrically conducting seal of small size and ofsimple structure for high pressure, high temperature dis-' chargedevices. is to provide an electrically conducting foil or strip memberhaving a low resistance to current flow for use in hermetic seals. Stillfurther objects and advantages of the invention will be apparent tothose skilled in the art from the following detailed description thereofand from the appended claims. We have discovered that the hightemperatur of the seal is due not only to the high temperature of thelamp envelope during operation of the lamp but also due to the heatdeveloped in the metal foil. which is about 20 microns, thick,

because of its appreciable resistance 'to the highcurrents. conductedthereby. By decreasing the electrical resistance of the foil the heatdeveloped in the seal is reduced and the lamp is capable of operating athigher pressures and envelopL temperatures without causing the formationof a layer oi'.,oxide on the exposed part ofthe metal conductor. Theresistance of the foil is decreased by increasing the cross-sectionalarea of v the center part'thereof. For example, one or more longitudinalmetal strips, foils or wires are mounted on the center portion of themain foil so as to make a good electrical contact therewith. Preferablythe auxiliary foil or' the like is welded or soldered to the main foil.An equivalent structure is a single foil which is thicker at the centerportion thereof than at the end portiona The thin end portions of thefoil both make an hermetic Joint withnthe vitreous part ofthe seal.

of this specification an embodiment of the in- I vention is shown, inwhich Fig. 1 is a side elevational, partly sectional view of the lamp ismade more complicated Another object of the invention ing therefrom andcontains two electrodes which are of the self-heating type. Each of saidelectrodes 5 consists of a perforated hollow body of high melting pointmetal, such as tungsten, having a material of high electron emissivity,such as thorium oxide, therein. During the operation of the lamp theelectrodes 5 are at an elevated, arc discharge sustaining temperature.Said envelope I has therein a starting gas, such as argon or neon, at apressure of about 1 or 2 centimeters and mercury in an amount sufficientto produce an unsaturated vapor atmosphere having a pressure of about 50atmospheres when the lamp is at operating equilibrium.

'Each of said seals 2 comprises a metal conducting element consisting ofa strip or foil 3 of high melting point metal, such as molybdenum, ametal wire 4, such as a tungsten wire, attached to said foil 3 andprojecting beyond the external end of the vitreous part of the seal. Asimilar metal wire 8 is attached to the opposite end of the foil 3 andterminates in the space defined by the envelope I where it supports theelectrode 5. The foil 3 is about 15 microns thick and is reinforced atthe central portion thereof by a strip 6 of high melting point metal,such as molybdenum. We prefer to weld the reinforcing strip 6 to thefoil 3 but it is possible to accomplish the objects of the invention bybringing the foil 3 and the strip 6 into contact with each other. Thestrip 6 increases the cross-sectional area of this part of theconducting element and substantially decreases the electrical resistancethereof to minimize the amount of electrical energy transformed intoheat by said foil 3. We have demonstrated that the above structure soeffectively reduces the internal resistance heating of the seal that thepart of the wire 4 protruding from the vitreous part of the seal isbelow the temperature at which an oxide coating forms thereon even whenthe current input of the lamp is about 500 watts and the length of thevitreous part of the seal is such that an oxide coating would form onthe protruding part of the wire 4 ii said strip 6 were omitted.

The-molybdenum foil 3 shown in Fig. 2 of the drawing is similar to thoseillustrated in Fig. 1 except that in this embodiment the foil 3 has aplurality of longitudinal strips I which are arranged in a staggered,overlapping pattern with a substantial distance between the strip I. Thehermetic juncture between the foil 3 and the vitreous part of the sealis located at the end portions of the foil 3 extending beyond the'stripsI and the portions of the foil 3 not covered by the strips 1.

A thicker molybdenum foil, such as a foil having a thickness of about 20microns, is used, when desired. In the fabrication of seals comprisingcomparatively thick foils it is desirable to apply a relatively highpressure to the vitreous part of the seal when in a softened conditionto force it into intimate contact with the foil. Even when thicker foilsare used the greater cross-sect onal area at the center portion of thefoil is desirable since this structure permits the use of higher currentdensities in the arc discharge and the use of comparatively narrowfoils.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A hermetic, electrically conducting seal comprising a vitreouselement having a low coefficient of expansion and an electricallyconducting element fused into'said vitreous element, said vitreouselement being an elongated cylindrical body, said conducting elementbeing a thin, fiat, elongated metal member mounted with its greatestdimension in the direction of the longitudinal axis of said vitreouselement, another element of electrically conducting material mounted onthe center portion of said first mentioned conducting element todecrease the electrical resistance of said center portion, the edgeportion of said first mentioned conducting element forming a hermeticjoint with said vitreous element.

2. An electrically conducting element for fusion with a vitreous part ofa hermetic seal,

said element consisting of a thin, elongated metal foil having aplurality of elongated strips mounted adjacent the center portionthereof in a staggered, overlapping pattern to minimize the heatgenerated in said foil by the passage of electrical energy therethroughand a pair of wires secured in opposing positions to the edge portion ofsaid foil.

LEOPOLD LINDWURM.

WOLFGANG THOURET.

